This invention relates to servo compensators and more particularly relates to a digitally-programmable proportional-derivative (PD) analog-signal servo compensator, especially for use in computer disk drives.
A typical servo control system is one in which the performance of an electric motor is continuously controlled by an electrical control signal. A servo compensator is connected between the control signal input and the motor, and electrical feedback reflecting the status of the motor is subtracted from the control signal. The compensator improves the servo feedback loop stability and otherwise improves servo system performance, e.g. by being tailored to compensate the inertia, vibration frequency, etc. of the particular motor. Computer disk drives include one servo system to control rotation of the disk and another servo system to control the actuator motor that determines the radial location on the disk of the read/write head.
Proportional-derivative, PD, compensators refer to those that produce an output signal portion that is proportional to the control signal and another output signal portion that is the derivative of the control signal. A further refinement to a PD compensator results in the generation of the yet another output signal portion that is the integral of the control signal, in which case it is said to be a PID compensator.
PD and PID circuits are often connected with and thus combined with standard filter circuits as separate blocks to form a servo compensator. A particularly effective filter circuit for limiting and defining the bandpass of the combination is a biquad filter having the S-plane transfer function. ##EQU1## and the combination of the filter with a PID circuit block to form a composite PID compensator has the form of ##EQU2##
For the PD composite compensator the transfer function is the same except the K.sub.I parameter has zero value.
Disk drive servo compensators have been implemented in the past either by analog circuits or by digital circuits. Digital signal processing are especially flexible and accurate. Because the algorithms are carried out in software, the user may change the algorithm variables at will. However, digital signal processing is very computation intensive and the interface code processing portion of the circuit has to schedule its time between servo and the rest of the disc drive functions. Also digital calculating methods require time for the processor to receive its input, process that input and send it to the output stage. For these reasons speed is not optimum in digital signal processing servo systems.
Analog disk drive servo compensators employ operational amplifiers, comparators, and many passive components. The sources of inaccuracy and drift in such components are legion; unpredictable amplifier offsets, non linearities with respect to current or signal amplitude, and parameter changes in all components with temperature. Although analog-signal servo compensators are fast, they lack the accuracy and flexibility of digital signal processing compensators.
It is therefore an object of the present invention to provide a digitally-programmable analog-signal servo compensator offering the superior speed of a conventional analog-signal compensator, and the stability , accuracy and flexibility approaching or equal to that of a conventional digital-signal-processing compensator.